Some olders versions of g++ do not implicitely link with libstdc++

https://github.com/intel/libvpl/commit/80ea0a5ebd25743c1cfe973e803245ba67d53b20 removed implicit linking with `libstdc++`.

Some older g++ versions do not implicitly link with `libstdc++`, causing it to not be listed in `vpl.pc`, resulting in
having a bunch of undefined C++ symbols for a C program linking to `libvpl` using `pkg-config`.

The work-around is to pass `-DCXX_LIB="-lstdc++" ` to `cmake`.

I found this issue upgrading from 2.14.0 to 2.17.0, compiling it with g++ 7.5.0 on openSUSE Leap 15.4.

Start by inspecting the CMake configuration that generates vpl.pc and reproduce the issue with g++ 7.5.0, CMake, and pkg-config. Confirm that a C program linking libvpl through pkg-config receives the libstdc++ dependency and no longer reports undefined C++ symbols.

Written by the indexing model from the issue text.